The present invention relates generally to systems for measuring the surface characteristics of traffic-bearing surfaces such as highways, roads, and airport runways. In particular, the present invention relates to a non-contact measuring system which accurately measures the characteristics of a surface over which it travels with reference to known, fixed, laser-generated reference planes.
Traffic-bearing surfaces such as highways, roads, and airport runways are subjected to wear from many sources including salt, studded tires, excessive weight, and friction, to name but a few. In addition, such surfaces may be subjected to settling or shifting of the underlying material, extreme temperature and weather variations, and other damaging effects. As a result, such surfaces often develop ruts, cracks, roughness, unevenness and other irregularities.
It is desirable to be able to quickly and accurately measure the characteristics of such surfaces in order to determine when, where, and to what extent the need for surface repair is indicated. Such measurements are also beneficial in determining whether newly laid surfaces correspond to specifications and guarantees, for example. It is also desirable to be able to perform such measurements on substantially the same surface locations at different times. By repeating measurements at the same locations on a surface, over a period of time, an indication of surface wear with respect to time is obtained. This information is most helpful, for example, in determining what types of surface materials to employ in various locations and for various uses.
Various systems are known for measuring the characteristics of road surfaces. However, these prior art systems suffer from various drawbacks which limit their usefulness.
For example, it is known to use conventional geodetic analysis, and to proceed from a plurality of points having known, fixed coordinates to obtain the coordinates of points on a traffic-bearing surface, and thereby determine the surface characteristics. However, this approach requires the surveying of a very large number of points, perhaps in the thousands, on the surface of interest. It is, therefore, a very time consuming and costly approach, especially where the surfaces to be measured extend for hundreds of miles.
Mobile measurement systems are also known. These generally involve propelling a measurement device along the surface being measuring. The measurement device itself may or may not directly contact the surface. Prior art mobile measurement systems generally have been able to overcome problems associated with the conventional geodetic approach.
For example, U.S. Pat. No. 3,266,302 issued Aug. 16, 1966 to Spangler discloses a mobile system for quickly measuring road surface irregularities using an accelerometer to measure the vertical acceleration of the sprung chassis of the measuring vehicle and an integrator to calculate vertical displacement of the road surface therefrom. A potentiometer attached to the sprung portion of the vehicle generates signals representative of vertical movement of the sprung portion.
U.S. Pat. No. 4,422,322 issued Dec. 27, 1983 to Spangler discloses an improvement on the '302 system wherein the road surface measurements are taken independent of changes in the horizontal velocity of the measuring vehicle. Vertical acceleration of the sprung chassis of the vehicle is still relied on as a value representative of the vertical displacement of the road surface.
However, propelling a measurement device along a surface being measured generates additional problems which have yet to be adequately solved in the known systems. When, as in the known systems, a vehicle having a sprung suspension carries the measuring device along the surface being measured, irregularities in the surface encountered by the wheels of the vehicle cause vibrations in the vehicle and vertical movements of the vehicle's sprung suspension due to compression and expansion of the vehicle's springs. These vertical movements cause the measured surface values to be inaccurate. This inherent inaccuracy is not eliminated by measuring the deviation in the vertical position of the sprung portion of the vehicle chassis with respect to another point on the same chassis because that point experiences similar vertical deviations. In addition, with known systems, there is no accurate way to determine that subsequent measurements of the same surface performed at a later time are made on substantially the same surface locations, or to register any deviation therefrom.
One approach to compensating for vertical deviations in the position of a measuring vessel in the field of oceanography has been to establish a horizontal laser reference plane. U.S. Pat. No. 3,890,840 issued June 24, 1975 to Malloy, discloses an underwater depth measuring system which employs a horizontal laser plane generated by a land-based laser as a vertical reference point for a ship carrying an underwater depth transducer. As the vertical position of the ship varies with respect to the reference plane due to surface disturbances, the signal from the underwater depth transducer is correspondingly adjusted. This system, however, does not address the problem of how to accurately perform subsequent measurements on the same surface at a later time at substantially the same locations as were previously measured. Nor does it address the problem of how to accurately maintain the vessel on a selected measurement path.